Control circuit



March 3, 1942. RAY 2,274,938

CONTROL CIRCUIT I Filed Aug. 29, 1938 Fae/me! 11 41/444 A. @Ay

INVENTOR A TTORNEY Patented Mar. 3, 1942 My present invention relates toelectrical control circuits and has for its chief object the provisionof a circuit in which an. electrically operated device is controlled byvariation of pressure across electrical contact members which arecontinuously in contact with one another during the operative cycle ofthe device.

A particular object oi this invention is the provision of a circuit inwhich current flow through an electromagnetic device is substantiallyconstantly varied by variation of pressure across a pair of metallicelectrical contact members which are in continuous contact with oneanother while so varying said current ilow as to move a membercooperable with said device between energized and unenergized positions;

The variable controlling pressure employed may be that generated by atemperature responsive bimetallic member, or in a iluid thermostat, orby other devices responsive to changes in conditions.

Other objects and advantages of my invention will be found in thedescription, the drawing and the appended claims.

For a more complete understanding of the invention, reference may be hadto the following detailed description and accompanying drawing, in whichFigure 1 is a diagrammatic view of apparatus embodying my invention: and

Figure 2 is a more or less diagrammatic view of a device shown only forthe purpose of explaining the operation of my invention, and forming nopart thereof.

Referring to Fig. 1 of the drawing, the numeral I ll denotes abimetallic member carrying a contact member I! cooperable with arelatively fixed contact member II mounted in an insulating member II.The'contact members I! and I! are connected by means of wires ll throughthe pressure on carbon granules 2! contained therein in contact with aconductive back plate 2. Electrical connection to the device may be madeby means of wires 21 and II.

In conventional control circuits that include the apparatusdiagrammatically shown in Fig. l, the circuit constants are such thatupon initial closing of the contact members the voltage acrm them dropssubstantially to zero with respect to that across the electromagneticdevice with the result that operation of said device is immediatelyelected. While the contacts are maintained in closed position,regardless of variation of pressure across them, the voltage across themremains substantially zero. when the contacts actually open, the voltageacross them rises to that of the source. It is usual in such a controlsystem to include magnetic or spring actuated "snap action means formoving the contact members rapidly to and from their closed position soas to prevent arcing and chattering of the contacts.'

In contradistinction to the above, by my invention, the circuitconstants are so arranged that upon initial closing of the contactmembers I! and It the voltage across them is still sub-. stantially thatof the source, or very high with respect to that across theelectromagnetic device II, with the result that said device remainsunoperated. I

Upon increase oi pressure being produced across the contact members byflexing oi the tromagnetic device. Upon decrease in pressure coil or anelectromagnetic device It to a source of electrical energy indicated bythe legend. The 1 electromagnetic device It is provided with an armatureIt cooperable with contacts I! and II to control an external circuit,which, by way of example, may include an electrically operated Y valve20 controlling fuel supply to a furnace,

indicated by the legend, for heating the space wherein the bimetallicmember is installed.

In Fig. 2, I have shown means, for more readily explaining the eilectproduced by the contact members I! and II of Hg. 1, which comprisesbimetallic member 2| operatively connected by a pivoted rod 22 to aplunger it alidable within a cylinder 24 of insulating material to varyacross the contacts, the voltage across them substantially constantlyincreases, with the result that a pointis reached at which theelectromagnetic device returns to unoperated condition while the contactmembers are still in mechanical contact.

Upon further possible movement of the contact members they may actuallyseparate, but without any eilect upon the operation of the controlcircuit.

For a clear understanding of the operation described above, referencemay be had to Fig. 2

.of the drawing the device shown therein being elective to vary thepressure between carbon granules, with the well known result that thevoltage across the device would be substantially theconstantlyvarledifitwereconnectedinacircuit including a source ofelectrical energy and a load or resistance.

For the purpose of my invention, I prefer to employ solid contactmembers made of such metals as platinum or silver. I also employ asource of voltage that is very low with respect to that commonlyemployed in control circuits. It may preferably be in the order of 0.25volt or less for best results with platinum or silver contact members.This source may conveniently be in the form of a thermoelectricgenerating device of low internal resistance, a D. C. or A. C. sourceprovided with a potentiometer, or an A. C. transformer wound for suchlow voltage. The electrically operated device that I employ has lowresistance or impedance, preferably in the order of one or two ohms,depending mainly upon the resistance or impedance of the source.

I have found that a control circuit employing the apparatus described inthe previous paragraph operates by variation of pressure across thecontact members; with the result that the contact members arecontinuously closed during the operative control cycle. The effectivevariation of contact resistance with variation of pressure is seeminglydue to the microscopic change in area of contact. This effect isproduced, for example, between fresh clean surfaces of platinum orsilver. The surface of at least one of the contact members is preferablyof spherical form as shown.

An outstanding advantage of the control circuit of my invention residesin the small differential of the conditioncontrolling the contactpressure required for operation of the controlled device. For example, Ihave found, by means of accurate recording instruments, that aconsistent 025 F. differential was maintained in the control of heatingapparatus by a bimetallic member so arranged as to move 0.0004 inch perdegree Fahrenheit. A sensitive millivoltmeter connected across thecontacts showed a substantially constant change throughout the variationof pressure across the contacts.

A further advantage is in the absence of arcing at the contacts whichobviously cannot occur under the constant variation of current whichoccurs in my control circuit.

The harmful effect of vibration of the contacts is also minimized, as ithas been shown that the initial making of contact has no effect on theoperation of the controlled device.

The load device of my circuit may be any form of sensitive electricallyoperated device having the required resistance characteristics.

The circuit of my invention has been found to have particular utility inconnection with the fluid pressure operated valve disclosed in mycopending application, Serial No. 222,673, filed August 2, 1938, alongwith the thermoelectric generating device disclosed in the patent ofWilliam 18.. Ray, No. 2,236,609, dated April 1, 1941. When such a valveis employed, no relay is required as the sensitive electrically operatedpilot valve of this valve may be connected directly in thethermostatically controlled circuit.

While I have shown the contact members arranged in series with thesource and electromagnetic device, it is obvious that control might alsobe elected by arranging the' source and load in a complete seriescircuit and shunting the contacts across the load and source, especiallyin connection with a thermoelectric generating device.

I do not claim any particular means for varying the pressure across thecontacts. as there are many well known means, in particular, conditionresponsive means, for effecting such variation.

No limitations upon my invention are intended other than are imposed bythe scope of the appended claims.

I claim as my invention:

1. In a system whereby the degree of a condition is automaticallycontrolled, in response to variations of said degree, in a narrow range:means for. changing the degree of said condition; a pair ofsmooth-faced, rigid, metallic contact members normally in engagementwithin said narrow range; a circuit, including a source of electricalenergy having an E. M. F. of a fraction of 9. volt, controlled by saidcontact members; a device, electrically operable between two controllingpositions dependent on the degree of energization thereof and biased toone of said positions, for controlling the means for changing the degreeof said condition and connected in said circuit; and means, movable inresponse to changes in said condition, for varying the pressure betweenthe contact members; the constants of the componentsof said circuithaving such values, with respect to the variation of the contactresistance of said members due to the variation in pressurebetween themproduced in said narrow range, that when said pressure has a low valuethe current flow in the circuit is insufficient to operate said deviceinto the other of said positions and only becomes suiiicient when thepressure exceeds said low value.

2. In a system whereby the temperature of a space is automaticallycontrolled in a narrow range in response to variation of saidtemperature: means for changing the temperature of said space; a pair ofsmooth-faced, rigid, metallic contact members normally in engagementwithin said narrow range of temperature; a circuit, including a sourceof electrical energy having an E. M. F. of a fraction of 9. volt,controlled by said contact members; a device, electrically operablebetween two controlling positions dependent on the degreeoi-energization thereof and biased to one of said positions, forcontrolling said temperature changing means and connected in saidcircuit; and means, movable in response to changes in the temperature ofsaid space, for varying the pressure between the contact members; theconstants of the components of said circuit having such values, withrespect to the variation of the contact resistance of said members dueto the variation in pressure between them produced in said narrow range,that when said pressure has a low value the current flow in the circuitis insufllcient to operate said device into the other of said positionsand only becomes suflicient when the pressure exceeds said low value.

WILLIAM A. RAY.

